Tore up assessment, that's telling! (9)
I believe the answer is:
narrating
'telling' is the definition.
('narrate' can be a synonym of 'tell')
'tore up assessment' is the wordplay.
'tore' becomes 'ran' (I've seen this in another clue).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'assessment' becomes 'rating' (rating is a kind of assessment).
'ran' backwards is 'nar'.
'nar'+'rating'='NARRATING'
'that's' acts as a link.
(Other definitions for narrating that I've seen before include "telling the tale" , "Providing commentary for a film, for example" .)